Heat olive oil (butter) over medium heat in a large pot (Dutch Oven). Add the onion and cook for 2 minutes, stirring frequently. Add the garlic and cook for 1 minute. Add tomatoes, salt, pepper, pesto (optional), oregano, basil, tomato paste and water. Bring to a boil, then reduce to a simmer. Add sweetener.
Cook on medium heat for 20 minutes, until the tomatoes are tender and cooker. Using an immersion blender, blend until smooth. Add the cream and feta cheese. Cook for 1 more minute.
Add more salt if needed. Serve warm.